Simplify vacuum_set_xid_limits()'s signature (minor refactoring)

Attached patch has vacuumlazy.c pass its VacuumParams state directly
to vacuum_set_xid_limits(), the vacuum.c function that figures out
which actual cutoffs for freezing should be used. The patch also makes
the function use output parameter symbol names that match those used
by its vacuumlazy.c caller.

The signature of vacuum_set_xid_limits() is probably going to gain
additional parameters before too long. In any case this seems like a
clear improvement.

-- 
Peter Geoghegan